The present invention relates to a method for producing a fluid dynamic pressure bearing supporting a rotation shaft without contact, and it is intended to increase a sealing effect sufficiently by forming a ferrosoferric oxide (Fe3O4) membrane especially on an inner peripheral surface and edge surfaces of a fluid dynamic pressure bearing made of sintered metal on which dynamic pressure generating grooves on an inner peripheral surface thereof are formed. These features increase a dimension accuracy and improve a dynamic pressure effect, increase strength of the sintered metal, and thus, increase durability and rotation accuracy of a motor and to achieve low noise.
In recent years, with high accuracy of AV equipment, OA equipment or the like, requests of rotation accuracy of motors such as especially a spindle motor for hard disc drives and optical disc drives as represented by DVD and CD, furthermore blue ray disc or the like, color wheel motors using projectors, LBP polygon mirror scanner motors, fan motors or the like, and of achieving low noise are being heightened remarkably, so that a fluid dynamic pressure type bearing supporting a motor shaft without contact are noticed in order to correspond these requests.
A fluid dynamic pressure type bearing can support a rotation shaft of a motor without contact by generating dynamic pressure by forming grooves on an inner peripheral surface and end surfaces of the bearing and filling lubricating oil in the grooves. As methods for producing these fluid dynamic pressure type bearings, in many cases, a blank (material) mainly of smelted material such as brass, stainless steel or the like is cut into a bearing shape, and then grooves are formed on an inner peripheral surface thereof by cutting or rolling.
As a blank (material) of a fluid dynamic pressure type bearing, in the case that brass is selected, though cutting performance is excellent, not only is there inferior durability by momentary contact with a motor shaft due to starting or stopping of a motor, an external load, vibration or the like, but also it is difficult to maintain available clearance due to variation of environmental temperature because of difference of expansion coefficient to a motor shaft made of stainless steel. Therefore, a use environmental temperature range of a motor is very narrow. In the case that stainless steel is selected as a blank (material) of the bearing, durability due to contact with a motor shaft is excellent and maintaining available clearance is possible because the difference of expansion coefficient is small, but there is a little problem in a cutting work.
In any blanks, in the case that a blank is cut into a bearing shape, remarkably high accuracy processing by an NC turning machine, machining or the like is necessary, because there is a little problem in mass production and in reducing the cost of production, a fluid dynamic pressure bearing possible to process a bearing in a near-net-shape and possible to produce in bulk and reduce the cost came to be considered.
In particular, in the case that sintered metal made in a powder metallurgical process is used as a blank, because a degree of freedom for selecting metal materials is high, not only iron system material that difference of expansion coefficient with a motor shaft made of stainless steel is small can be selected, but also the near-net-shape process is performed, so that process in which a blank being a difficult cutting material is cut into a bearing shape can be omitted, as a result it is possible to contribute to mass-production and decreasing the cost.
However, on the other hand, because the power sintered metal blank is a porous solid made of powder metal, in the case that this is used as a fluid dynamic pressure bearing, there are fatal problems such that dynamic pressure worked on an inner peripheral surface of the bearing is leaked and an oil surface necessary to generate stable dynamic pressure falls down because the oil filled with regular amount into a gap between the motor shaft and the inner peripheral surface of the bearing penetrates into the porous solid.
In fact, in a usual process, it is impossible to eliminate existence of communication pores due to air gaps between metal particles constituting powderintered metal material perfectly, stiffness of the bearing is decreased by that dynamic pressure generated on an inner periphery surface of the bearing leaks through the communication pores, so that there are large problems such that rotation accuracy and further a life-span of a motor is affected negatively, and further that control of oil quantity is very difficult.
Therefore, a dynamic pressure bearing such that air gaps are sealed by impregnating a powderintered metal material with resin has already been proposed (JP 8-221897 A). In the case of pure iron, air gaps are sealed by impregnating a powderintered metal material with resin and then a coat is formed thereon in order to compensate for lack of surface stiffness. Furthermore, after air gaps are sealed by impregnating a powderintered metal material with substance such as resin, metal or glass, it is considered that opening area of air gaps are contracted by giving a shot blast using metal particles or resin particles on a surface of a bearing.
Besides, a fluid dynamic pressure bearing such that surface roughness, corrosion resistance and abrasion resistance are attempted to be improved according to sealing pores on a surface by giving steam treatment to the powderintered metal material has been proposed (JP 2007-57068 A).
However, after cutting a blank (material) mainly of smelted material such as brass or stainless steel into a bearing shape, in the case of processing grooves on an inner peripheral surface by cutting or rolling, since it is necessary to be performed with an extremely high degree of accuracy by a NC lathe, it cannot be avoided to become high-cost. In the case of impregnating the powderintered metal material with resin, it tends to stay a resin impregnation material on a surface of a bearing material in a usual process.
There remain further problems such that dimension accuracy tends to have a bad influence and it is difficult to apply a coat on the resin surface when the resin impregnation material stays on the surface of the bearing material, that coating treatment as a post-processing tends to be imperfect when pores before impregnating resin is large, and further that metal erosion by staying coating liquid tends to take place because it is difficult to remove coating liquid in the pores perfectly though coating treatment takes place on a surface of a bearing material in order to supply a want of a surface strength in pure iron.
Besides, in the case that resin impregnation treatment takes place, though a cleaning work has to be taken place in every bearing material, because bruises occur due to collision between the bearing materials themselves when the bearing materials are cleaned in bulk, every bearing material has to be cleaned individually to avoid the bruises, so that the cost increases remarkably. Further, expansion or retraction occurs by reacting the impregnated resin with fluid such as lubricant oil used in a fluid dynamic pressure bearing unit, and as a result it is possible to cause concern on quality and accuracy.
Furthermore, in the case that a shot blast processing takes place on a surface of a bearing after sealing pores by impregnating resin or the other substance into a powderintered metal material, roughness of a surface of a bearing generally gets worse, there are problems such that not only is it unsuitable as a fluid dynamic pressure bearing, but also costs increased because a quality of products is quite variable due to worse dimensional accuracy, and a cleaning process for removing shot powdertaying on the bearing material is necessary separately.
Moreover, in the case of a fluid dynamic pressure bearing that surface roughness, corrosion resistance and abrasion resistance are improved by sealing pores on a surface of bearing due to giving steam treatment to a powderintered metal material, since diameters of pores on a surface and an interior portion of the powderintered metal material have a large variation generally and there is remarkably variation in a sealing pore effect, it is difficult to maintain a stable dynamic pressure and it is necessarily the case that a sufficient function can bring out as a dynamic bearing.